Elastomeric thin-walled articles, such as gloves for surgical use, are generally manufactured from natural rubber, usually by dipping a glove former into a vessel containing natural rubber latex. However, problems have been encountered with natural rubber surgical gloves since small quantities of proteins may leach from the glove.
As a result, attempts have been made to manufacture surgical gloves from synthetic materials such as polyurethane. For example, U.S. Pat. No. 4,463,156 to McGary, Jr., describes, in Example 14, the manufacture of a mixed diol, 4,4'-diphenylmethane diisocyanate polyurethane. A 20% solids solution of the polyurethane was prepared and a glove produced by dipping a glove former into the suspension. However, such polyurethane suspensions tend to be fairly unstable during storage and therefore do not lend themselves to production processes.
European Patent Application No. 0413467 discloses polyurethane condoms which are manufactured by dipping a suitably shaped former into an organic solvent solution of a polyurethane. However, such processes contain undesirable solvents that give rise to increased costs and problems with waste products.
Another major problem associated with the use of elastomeric materials, such as polyurethane, is the presence of voids and microporosities that reduce elastomer strength and elongation. Elastomer, or polymer, strength and elongation depend on the concentration and intrinsic strength of the polymer and is a function of both the structure of the polymer and its molecular weight.
The absence of voids in protective coatings and for articles, such as condoms and gloves, is also essential to prevent the transmission of viruses and causing other injuries. Voids can develop where polymerization or coagulation of particles is interrupted by solid impurities that are not wetted by the curing elastomer, as is the case with the macromolecules forming latexes. Likewise, although the detailed microscopic mechanisms of systems undergoing phase transitions are not completely understood, especially near phase transitions where large numbers of particles act coherently, voids tend to develop in the interstitial spaces between relatively large elastomer molecules when articles are formed by the evaporation of liquids, such as solvents or carrier liquids. As a result, polymer particle size and size distribution are important and limiting characteristics affecting film formation.
The drying of polymer coatings is commonly characterized by four stages: ordering, deformation, coalescence and interdiffusion. Ordering occurs as water or other liquids evaporate and polymer particles approach each other to form a close-packed array. Deformation occurs when the particles subsequently deform to fill the interstitial space between them but remain physically separated by hydrophilic layers consisting of water and surfactant material. Coalescence occurs as the hydrophilic membranes begin to break up and the particles come into contact with one another. Finally, interdiffusion occurs when the polymer chains diffuse across particle boundaries. Interdiffusion leads to mechanical strength and the loss of all notice of the initial particles. However, the described drying mechanism previously precluded their use because the relatively large interstitial spaces between the particles trapped large amounts of liquids, compared to the amount of liquid in the smaller interstitial spaces between small particles. That resulted in poor drying of the polymer film and, thus, poor mechanical characteristics to the product. Prior art compositions exhibit less advantageous mechanical properties as particle size increases. In addition, the compositions tend to form undesirable agglomerations after a prolonged period of storage.
Sadowski et al. (J. S. Sadowski, B. Martin and D. D. Gerst, "Polyurethane Latexes for Coagulation Dipping," Elastomerics, August 1978, 17-20) recognized that particle size is, relatively, one of the most critical factors in the selection of aqueous-based dispersions. Sadowski et al. disclosed that polyurethane gloves may be made by the "Anode" process that dips a former into a coagulant bath and then dips the former into a polyurethane latex with a particle size between 0.06 to 0.3 .mu.m. With particle sizes above about 0.3 .mu.m, the dispersions become increasingly less suitable for the production of thin coatings because dispersions with a fairly high average particle size tend to settle out. In accordance with the prior art, it is therefore preferable to employ a dispersion having an average particle size of no more than 0.3 .mu.m.
To date, polyurethane gloves manufactured using Sadowski et al.'s method or any other coagulation dipping method have never been made commercially available. Thus, there has been a long felt want for a commercially viable process for the dispersion dipping of polyurethane articles and polyurethane gloves in particular.